Waterjet systems have a cutting head that produces a high-velocity waterjet that can be used to cut or pierce workpieces composed of a wide variety of materials. Abrasives can be added to the waterjet to improve the cutting or piercing power of the waterjet. Adding abrasives results in an abrasive-laden waterjet referred to as an “abrasive waterjet” or an “abrasive jet.” The diameter of holes pierced and the kerf width of slots cut with the abrasive waterjet are proportional to the jet stream diameter. Reducing the hole diameter and kerf width can be achieved by reducing the size of the corresponding abrasive jet nozzle. Under certain circumstances, however, reducing the jet nozzle size may be difficult.
Certain materials, such as composite materials and brittle materials, may be difficult to pierce with an abrasive jet. An abrasive jet directed at a workpiece composed of such material strikes a surface of the workpiece and begins forming a cavity. As the cavity forms, a hydrostatic pressure may build within the cavity resulting from the conversion of the kinetic energy of high-speed water droplets into potential energy. This hydrostatic pressure may act upon sidewalls of the cavity and negatively impact the workpiece material. In the case of composite materials such as laminates, such hydrostatic pressure may cause composite layers to separate or delaminate from one another as the hydrostatic pressure exceeds the tensile strength of the weakest component of the materials, which is typically the composite binder. In the case of brittle materials such as glass, polymers, and ceramics, the hydrostatic pressure may cause the material to crack or fracture if the hydrostatic pressure acts upon intergranular cracks and/or micro fissures in the material or simply exceeds the tensile strength of the weakest material in the specimen being cut. Other aspects or effects of the abrasive jet other than the hydrostatic pressure may, in addition or as an alternative to the hydrostatic pressure, cause or result in damage to the material during abrasive jet piercing operations.
Conventional techniques for mitigating piercing damage to materials include pressure ramping and vacuum assist devices. Pressure ramping can involve using a reduced water pressure to form the waterjet and ensuring that abrasives are fully entrained in the waterjet before the hydrostatic pressure reaches a magnitude capable of causing damage to the material being pierced. A vacuum assist device can be used to draw abrasive into a mixing chamber of a waterjet cutting head prior to the arrival of water into the mixing chamber. Such a technique can prevent a water-only jet from striking the surface of the material.